Preparation of deashed high solid concentration coal-water slurry

ABSTRACT

This process, which comprises subjecting coal to gravity classification to classify into a low ash coal, a middle ash coal and a high ash coal; deashing the middle ash coal by flotation; and using the deashed middle ash coal together with the low ash coal as the materials for producing a coal-water slurry, provides a low ash content and high solid content coal-water slurry in a high coal recovery as well as economically. This coal-water slurry is able to handle, store and burn as are fuel oils.

BACKGROUND OF THE INVENTION

The present invention relates to a process for preparing a coal-waterslurry which is nearly the same, to handle, store and burn, as fueloils, in particular a process for the preparation of a coal-water slurryhaving low ash content and high solid content with a high coal recoveryas well as economically, regardless of the amount of ash content ofparent coal.

It is well known to prepare a high solid content coal-water slurry bymixing a ground coal with water or wet grinding a mixture of crushedcoal and water. In these preparations it is desirable to reduce theamount of ash which is mingled in the slurry accompanied by the coal asmuch as possible. The coal contains more or less of ash comprising Al₂O₃, SiO₂, FeS and the like. Mingling of ash in the coal-water slurry isdisadvantageous in that when said slurry is burnt, not only is abrasionof the boiler walls brought about, but also the heating value of saidslurry is lowered.

Under these circumstances, in the preparation of a high concentrationcoal-water slurry there has hitherto been employed a process whichcomprises subjecting a relatively coarse grain-sized parent coal togravity classification to thereby obtain a low ash coal whose ash amountis in the allowable range, and grinding said low ash coal alone toobtain a coal-water slurry, or a process which comprises grinding thewhole amount of a parent coal itself and thereafter subjecting it todeash treatment for obtaining a low ash coal. However, the formerprocess is defective in that a certain amount of combustible matter isnot transferred to said low ash coal and is not utilized as thecombustible component for slurry, and therefore the coal recovery islow. On the other hand, the latter process is high in coal recovery ascompared with the former process, but is economically unprofitable inthat because a large amount of coal is subjected to deash treatment,there must be used a large-sized deash equipment and accordingly thecosts of equipment and working the apparatus are high.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a process which iscapable of eliminating the above mentioned problems and producing adeashed high concentration slurry with a high coal recovery as well asin an economical manner even when coal having a relatively large ashcontent is used as the parent coal.

In order to achieve the above mentioned object, the present inventionprovides a process for preparing a deashed high solid concentrationcoal-water slurry comprising the steps of (a) subjecting coal having aparticle size of under 100 mm under coal to gravity classification toclassify said coal into a low ash coal having a specific gravity of 1.4or less, a high ash coal having a specific gravity of 1.5 or more and amiddle ash coal having a specific gravity higher than that of the lowash coal and lower than that of the high ash coal; (b) grinding saidmiddle ash coal so that 50% or more thereof has a particle size of under200 mesh and adding water thereto for preparing a middle coal slurryhaving a solid concentration of 5-25%; (c) subjecting this middle coalslurry to flotation for obtaining a froth having a reducing ash content;and (d) mixing the low ash coal obtained in the preceding step (a) withsaid froth, and therafter grinding said coal mixture so that 50% or morethereof has a particle size under 200 mesh or grinding said low ashcoal, prior to mixing it with the froth, so that 50% or more thereof hasa particle size under 200 mesh and thereafter mixing this with saidfroth.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a flow diagram illustrating one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Generally speaking, the specific gravity of coal depends upon the amountof ash contained therein. Accordingly, it is possible to classify thecoal optionally in respect of ash content by subjecting relativelycoarse particle-sized coal to gravity classification. The first step ofthe present invention classifies, using the gravity classification, coalhaving a particle size under 100 mm into 3 fractions, namely the low ashcoal whose specific gravity is 1.4 or less, the high ash coal whosespecific gravity is 1.5 or more and the middle ash coal whose specificgravity is higher than the low ash coal and lower than the high ashcoal. In this connection, attention should be given to the fact that theupper limit of the specific gravity value of the coal classified as thelow ash coal in the present invention and the lower limit of thespecific gravity value of the coal classified as the high ash coal inthe present invention may be established optionally in the range of 1.4or less and in the range of 1.5 or more, respectively, according to thetarget ash amounts of the final product coal-water slurry, and the upperlimit specific gravity value of the coal classified as the middle ashcoal is altered by said established value. For instance, in case thecoal having a specific gravity of 1.35 or less is classified as a lowash coal and the coal having a specific gravity of 1.6 or more as a highash coal according to gravity classification, the coal having a specificgravity of 1.35-1.6 is classified as a middle ash coal from the parentcoal.

The middle ash coal classified from the parent coal is dewatered andcrushed to a particle size of under 3 mm, and mixed with under-sizeparticles of the screen installed prior to the gravity separator. Themixture is wet ground so that 50% or more thereof, preferably 70% ormore thereof, may have a particle size of under 200 mesh. The wetgrinding can be effected in the presence of a dispersing agent or in theabsence of said agent. When using the dispersing agent, its amount is inthe range of 0.01-3%, preferably in the range of 0.1-1% based on thecoal weight. The wet ground middle ash coal is then added with water,whereby there is prepared a middle ash coal slurry having a solidconcentration suitable for the flotation effected in the next step, thatis 5-25%, preferably 5-15%.

Flotation of the middle ash coal is effected in the presence of acollector whose amount is 0.05-0.35, preferably 0.1-0.3% based on thecoal weight and in the presence of a frother whose amount is 0.02-0.2%,preferably 0.03-0.15%, and thus there is obtained a deashed coal slurrycontaining the ash in an amount less than the ash contained in themiddle ash coal slurry before flotation, namely, froth. The collectorsusable in the flotation step include diesel fuel oil, kerosine and thelike, and the frothers usable in the present invention include methylisobutyl carbinol (MIBC), pine oil and the like. The froth obtained bythe flotation step can be dewatered as occasion demands.

The coal classified as a low ash coal from the parent coal in thegravity classification step is crushed, thereafter mixed with said frothor dewatered froth and then said mixture is wet ground typically so that50% or more of the mixed low ash coal, preferably 70% or more thereof,may become of a particle size under 200 mesh. The low ash coal ispreviously subjected to dry or wet grinding prior to its mixing with thefroth, and the thus ground low ash coal may be mixed with the froth ordewatered froth obtained from the flotation step.

Regardless of whether the low ash coal is subjected to wet or drygrinding, and regardless of whether said grinding is effected before orafter it is mixed with the froth, the ground low ash coal is mixed withthe froth to thereby prepare a final product of the present invention,namely a coal-water slurry. Generally speaking, the product slurrydesirably contains a dispersing agent in the range of 0.01-4%,preferably 0.1-2% based on the coal weight, for the purpose of enhancingthe stability of the product slurry. The dispersing agents used in thepresent invention include anionic, cationic and nonionic surface activeagents, and they may be used singly or in combinations which is selectedproperly according to the kind of coal used. Citing concrete examples ofeach surface active agent, the anionic surface active agents includessalts of sulfuric acid esters of fatty oils, salts of sulfuric acidesters of higher alcohols, salts of sulfuric acid esters of ethers,salts of sulfuric esters of olefins, alkyl allyl sulfonic acid salts,sulfonic acid esters of dibasic acid ester, salts of dialkyl sulfosuccinic acid, acylsarcosinate, salts of alkyl benzene sulfonic acid,salts of alkyl sulfonic acid esters, salts of dialkylsulfo succinic acidesters, alkyl acid or/and maleic anhydride copolymer, polycyclicaromatic sulfonate, formalin compound and the like. As cationic surfaceactive agents, there can be enumerated alkyl amine salts, quaternaryamine salts and the like. The nonionic surface active agents used hereininclude polyoxy alkyl ethers, polyoxy ethylene alkyl phenol ethers,oxyethylene-oxypropyrene block polymers, polyoxyethylene alkyl amines,sorbitan fatty acid esters, polyoxy ethylene sorbitan fatty acid estersand the like.

The drawing is a flow diagram illustrating one embodiment of the processaccording to the present invention. In the drawing, parent coal having aparticle size under 100 mm is subjected to screening by means of ascreen 11 of 0.1-20 mm, preferably 0.5-2 mm mesh, and thereafteroversize particles are supplied into a gravity separator 15 wherein thesupplied parent coal is classified a low ash coal having a specificgravity of 1.4 or less, a high ash coal having a specific gravity of 1.5or more, and a middle ash coal whose specific gravity is higher than thelow ash coal and lower than the high ash coal. The high ash coal isrejected as refuse from the separator 15. Concerning the under-sizeparticles of the screen 11, it is preferable to recover the coal byseparating the slime contained therein by using a separator 19, andtreat it in admixture with the middle coal obtained from the gravityseparator 15 after the middle coal has been subjected to treatment inthe dewaterer 27' and crusher 29'.

The admixture is supplied in a wet mill 21 together with water in anamount sufficient to obtain a coal-water mixture having a solidconcentration 5-60%, preferably 10-50%, and in this mill, and the sameis ground so that 50% or more, preferably 70% or more of the coal maybecome less than 200 mesh, whereby there is prepared a slurry. Theslurry obtained from the wet mill 21 is then sent to a conditioner 23and is added with water, whereby the solid concentration of said slurryis controlled in the range of 5-25%, preferably 5-15% which is suitablefor flotation. The slurry supplied from the conditioner 23 to aflotation machine 25 is subjected to flotation in the presence of acollector whose amount of 0.05-0.35%, preferably 0.1-0.3% based on thecoal weight and in the presence of a frother whose amount is 0.02-0.2%,preferably 0.03-0.15% based on the coal weight, and then a froth havinga deashed coal concentration of 15-30%, preferably 18-25% is recoveredfrom the flotation machine.

The coal classified as a low ash coal by means of the gravity separator15 is dewatered by a screen 27, thereafter is supplied in a crusher 29so as to crush generally to a particle size of under 30 mm, preferablyunder 5 mm, and then is mixed with the froth coming from the flotationmachine. This mixture is then supplied in a wet mill 31 and is ground sothat 50% or more, preferably 70% or more of the low ash coal may become200 mm under in the presence of a dispersing agent whose amount maytypically be 0.01-4%, preferably 0.1-2% based on the coal weight. Bythis grinding, there is prepared a final product, namely a deashed highsolid concentration coal-water slurry. In the coal-water slurry preparedaccording to the process of the present invention, typically, the solidconcentration is at least 60%, and at least 50%, preferably 70% of thecoal contained in the slurry has a particle size of 200 mesh under.However, the solid concentration of the slurry can be changed optionallyin the usual manner well known to those skilled in this art ofcontrolling the amount of water used in the process or adding anoptional dewatering step. Likewise, the particle size of the coal in theslurry may be changed optionally by controlling the grinding degree ofthe coal.

EXAMPLE 1

By using parent coal having a particle size of 20 mm or less and an ashcontent of 8.2%, there was prepared a deashed high concentration slurryaccording to substantially the flow shown in the drawing, but withoutusing a slime separator 19.

1570 g of the parent coal (A) was screened by means of a 0.5 mm-meshscreen to obtain 94 g (6.0 wt. %) of undersize particles whose ashcontent is 15.0% and 1476 g (94.0 wt. %) of oversize particles whose ashcontent is 7.8%.

These oversize particles was subjected to the gravity classification.The particles whose specific gravity is 1.6 or more and ash content is57.7% or more were separated as refuse, and the remainder was separatedinto 1243 g (79.2 wt. %) low ash coal having a specific gravity of 1.4or less and an ash content is 4.6%, and 157 g (10.0 wt. %) middle ashcoal having a specific gravity of 1.4-1.6 and an ash content of 9.0%.

Said middle ash coal was mixed with said undersize particles to obtain251 g (16 wt. %) of mixture having an ash content of 11.3%. Water wasadded to this mixture to regulate the solid concentration to be 50%, andthereafter was ground in a wet mill so that 75% of the coal may become aparticle size of 200 mesh (74 μm) under. Water was added again to thisground matter to regulate the solid concentration to be 15 wt. %,thereafter a collector (fuel oil) in an amount of 0.1% per coal and afroth (MIBC) in an amount of 0.1 wt. % per coal were added to same forflotation in order to remove 31 g (2.0 wt. %) of tail whose ash contentis 50 wt. %; and thus 220 g (14.0 wt. %) of a flotation froth having anash content of 5.8% was recovered. The solid concentration of said frothwas 26 wt. %.

On the other hand, said low ash coal was subjected to crushing so that90% thereof may become 3 mm under. This crushed low ash coal was mixedwith said flotation froth, further a dispersing agent was added theretoin an amount of 0.8 wt. % based on the coal weight, and was subjected towet grinding so as to obtain a high concentration slurry having a solidconcentration of 70 wt %. This high concentration slurry was observed tohave an ash content of 4.8% and to have yield of 93.2% and coal recoveryof 96.6%.

EXAMPLE 2

This example prepared a deashed high concentration slurry in accordancewith the flow stated in Example 1 except that a dewatered step wasprovided on the downstream side of a flotation step.

800 g of the parent coal (A) having a particle size of 10 mm or less andan ash content of 8.2% was screened by means of a 0.5 mm-mesh screen toobtain 101 g (12.6 wt. %) of undersize particles having an ash contentof 10.0% and 699 g (87.4 wt. %) of oversize particles having an ashcontent of 7.9%. These oversize particles was subjected to the gravityclassification, and 52 g (6.5 wt. %) of oversize particles having aspecific gravity of 1.6 or more and an ash content of 52.2% wasseparated as refuse. Thereafter, the remainder was further separatedinto a 487 g (60.9 wt. %) of low ash coal having a specific gravity of1.4 or less and an ash content of 3.1% and 160 g (20.0 wt. %) of middlecoal having a specific gravity of 1.4-1.6 and an ash content of 8.8%.

After dewatering and crushing, this middle ash coal was mixed with theundersize particles to obtain 261 g (32.6 wt. %) of a mixture having anash content 9.3 %. Water was added to this mixture so that the solidconcentration may become 45%, and thereafter was subjected to grindingin a wet mill so that 75% of the coal may have a particle size of 200mesh (74 μm) under. Water was added again to this ground matter toregulate the solid concentration to be 10 wt. %, thereafter a collector(fuel oil) in an amount of 0.1 wt. % per coal and a frother in an amountof 0.04 wt. % per coal were added to same for flotation to remove 17 g(2.1 wt. %) of tail having an ash content of 37.4 wt. %, therebyrecovering 244 g (30.5 wt. %) of a flotation froth having an ash contentof 7.3%. The solid concentration of this flotation froth was 22 wt. %,and concentrated by means of a dehydrater.

On the other hand, said low ash coal was subjected to crushing so that90% thereof may become 3 mm under. This crushed low ash coal was mixedwith said dewatered flotation froth, further a dispersing agent wasadded thereto in an amount of 0.7 wt. % per coal, and was subjected towet grinding. Thus, a high concentration slurry having a solidconcentration of 72 wt. %. This high concentration slurry was observedto have an ash content of 4.5% and further to have yield of 91.4% andthe coal recovery of 95.1%.

EXAMPLE 3

In this example, there is shown the procedure of dry grinding a coalclassified as a low ash coal prior to mixing with a froth coming from aflotation step.

A parent coal (B) having a particle size of 60 mm or less and an ashcontent of 24.6% (1100 g) was screened by means of a 0.5 mm-mesh screento obtain 88 g (8.0 wt. %) of undersize particles having an ash contentof 19.0% and 1012 g (92.0 wt. %) of oversize particles having an ashcontent of 25.1%.

These oversize particles were subjected to the gravity classification toseparate 125 g (11.4 wt. %) of oversize particles, whose ash content is75.0%, as refuse. Thereafter, the remainder was separated into 724 g(65.8 wt. %) of low ash coal having an ash content of 17.5% and 163 g(14.8 wt. %) of middle ash coal having an ash content of 22%.

After dewatering and crushing, this middle ash coal was mixed with theundersize particles to thereby obtain 251 g (22.8 wt. %) of a mixturehaving an ash content of 21.0%, Water was added to this mixture foradjusting the solid concentration to be 50%, and thereafter was groundin a wet mill so that 80% of the coal may become 200 mesh (74 μm) under.Water was added again to this ground matter to regulate the solidcontent to be 15 wt. %, and a collector (residual oil) in an amount of0.2 wt. % per coal and a frother (MIBC) in an amount of 0.05 wt. % percoal were added thereto for the practice of flotation, whereby 16 g (1.5wt. %) of tail having an ash content of 75 wt. % was removed and 235 g(21.4 wt. %) of a flotation froth having an ash content of 17.3% wasrecovered.

On the other hand, 724 g of the above mentioned low ash coal was crushedso that 90% thereof may have a particle size of 3 mm under. Insuccession, the same was subjected to a grinder so that 80% of the coalmay have a particle size of 200 mesh under. This ground low ash coal andthe aforesaid flotation froth were mixed by means of a mixer, andsimultaneously a dispersing agent was added thereto in a amount of 0.8%per coal to thereby obtain a high concentration slurry having a solidconcentration of 67%. This high concentration slurry was observed tohave an ash content of 17.5%, a yield of 87.1%, and a coal recovery of95.4%.

EXAMPLE 4

In this example, there is shown the procedure of dry grinding a coalclassified as a low ash coal prior to mixing with a dewatered frothcoming from a flotation step.

530 g of a parent coal (B) having a particle size of 35 mm or less andan ash content of 24.6% was screened by means of a 0.5 mm-mesh screen toobtain 75 g (14.2 wt. %) of undersize particles having an ash content of17.8% and 455 g (85.8 wt. %) of oversize particles having an ash contentof 25.7%.

These oversize particles were subjected to the gravity classification toseparate 80 g (15.1 wt. %) of oversize particles having an ash contentof 86.1%, as refuse, thereafter the remainder was separated into 202 g(38.1 wt. %) of low ash content having an ash content of 7.3% and 173 g(32.6 wt. %) of middle ash coal having an ash content of 19.6%.

After dewatering and crushing, this middle ash coal was mixed with theaforesaid undersize particles to thereby obtain 248 g (46.8 wt. %) of amixture having an ash content of 19.1%. Water was added to this mixturein order to adjust the solid concentration to be 50%, and thereafter wasground in a wet mill so that 80% of the coal may become 200 mesh (74 μm)under. Water was added again to this ground matter to regulate the solidconcentration to be 10 wt. %, and thereafter a collector (residual oil)in an amount of 0.2 wt. % per coal and a frother (MIBC) in an amount of0.05 wt. % per coal were added thereto for the practice of flotation,whereby 15 g (2.8 wt. %) of tail having an ash content of 57 wt. % wasremoved and 233 g (44.0 wt. %) of a flotation froth having an ashcontent of 16.6% was recovered and dewatered.

On the other hand, 202 g of the aforesaid low ash coal was subjected tocrushing so that 85% of the coal may have a particle size of 3 mm under.In succession, the same was subjected to grinding by means of a grinderso that 75% of the coal may have a particle size of under 200 mm. Thisground low ash coal and the dewatered flotation froth were mixed bymeans of a mixer, and simultaneously a dispersing agent was addedthereto in an amount of 0.6% per coal to thereby obtain a high solidconcentration of 71%. This high concentration slurry was observed tohave an ash content of 12.3%, a yield of 82.1% and a coal recovery of99.5%.

According to the process of the present invention, the parent coal canbe classified into low ash coals, middle ash coals and high ash coals(refuse) and the middle coals alone are deashed by flotation, so thatthe load in the flotation step can be reduced, in addition as thedeashed middle ash coals can be utilized, together with low ash coals,as the materials for preparing a coal-water slurry, the coal recoverycan be improved. It is safe to say that the process according to thepresent invention is exceedingly useful as a process for preparing acoal-water slurry replaceable for fuel oil.

We claim:
 1. A process for preparing a deashed, coal-water slurry containing at least 60 wt. % of coal solids, at least 50 wt. % of said coal solids having a particle size of less than 200 mesh, which comprises the steps of:(a) subjecting coal particles having a particle size of less than 100 mm to gravity classification and separation, and separately recovering three fractions as follows:(i) particles of low ash coal having a specific gravity of 1.4 or less, (ii) particles of high ash coal having a specific gravity of 1.5 or higher, and (iii) particles of middle ash coal having a specific gravity of higher than that of said low ash coal and lower than that of said high ash coal; (b) grinding said middle ash coal fraction (iii) so that 50 wt. % or more of said middle ash coal fraction has a particle size of less than 200 mesh and adding water thereto to obtain a middle ash coal-water slurry containing from 5 to 25 wt. % of middle ash coal particles; (c) subjecting said middle ash coal-water slurry obtained in step (b) to flotation and separately obtaining(iv) a froth of coal particles having a reduced ash content, and (v) tailings having an increased ash content; and (d) mixing the froth (iv) obtained in step (c) with the low ash coal fraction (i) obtained in step (a) to obtain a mixture containing at least 60 wt. % of coal particles, and subjecting the mixture to wet grinding so that 50 wt. % or more of the coal contained in the mixture has a particle size of less than 200 mesh, whereby to obtain said deashed, coal-water slurry.
 2. A process according to claim 1, wherein the middle ash coal is subjected to wet grinding in the step (b).
 3. A process according to claim 1, wherein the low ash coal is subjected to dry grinding prior to mixing with the froth in the step (d).
 4. A process for preparing a deashed, coal-water slurry containing at least 60 wt. % of coal solids, at least 70 wt. % of said coal solid having a particle size of less than 200 mesh, which comprises the steps of:(a) subjecting coal particles having a particle size of less than 100 mm to gravity classification and separation, and separately recovering three fractions as follows:(i) particles of low ash coal having a specific gravity of 1.4 or less, (ii) particles of high ash coal having a specific gravity of 1.5 or higher, and (iii) particles of middle ash coal having a specific gravity of higher than that of said low ash coal and lower than that of said high ash coal; (b) discarding said high ash coal fraction (ii), (c) grinding said middle ash coal fraction (iii) so that 70 wt. % or more of said middle ash coal fraction has a particle size of less than 200 mesh and adding water thereto to obtain a middle ash coal-water slurry containing from 5 to 15 wt. % of middle ash coal particles; (d) subjecting said middle ash coal-water slurry obtained in step (c) to flotation and separately obtaining(iv) a froth containing from 18 to 25 wt. % coal particles having a reduced ash content, and (v) tailings having an increased ash content; and (e) discarding said tailings; (f) crushing said low ash coal fraction (i) to a particle size of less than 5 mm; and (g) mixing the froth (iv) obtained in step (c) with the crush low ash coal fraction (i) obtained in step (f) and with a dispersing agent to obtain a mixture containing at least 60 wt. % of coal particles, and subjecting the mixture to wet grinding so that 70 wt. % or more of the coal contained in the mixture has a particle size of less than 200 mesh, whereby to obtain said deashed, coal-water slurry. 